1. Field of the Invention
The present invention relates to reinforced balloons and to several methods for creating composite films of organic polymers and inorganic additives on a nanometric scale which are compounded with polymers to be used to form angioplasty ballons to increase their strength. The super balloons or reinforced balloons are created with carbon nanotubes, clay platelets or ceramic, e.g. alumina, fibers which are included in the balloon material with a polymer.
Angioplasty balloons are required to be able to withstand very high pressures, which force the balloon's surface against various vessel tissues and deposits representing a range of viscoelastic characteristics, and include some very hard and rough surfaces. As the balloons must be thin-walled to collapse into a small profile (cross-section) for introduction to the target area, the balloons must be made extremely strong and puncture resistant. The balloons also must expand in a predictable manner when the internal pressure is beyond the nominal value where the cross-section is rated. In addition, balloon catheters are also used to deploy metallic stents within a constricted vessel. Stents are expandable wire or flat metal mesh devices that help retain proper vessel lumen after dilation. In this application, the balloon must come in contact with a metallic mesh that may inflict damage to the balloon. To meet these exacting requirements the present invention teaches several methods of creating composite films of organic polymers and inorganic additives, including carbon nanotubes, nanoclays, and ceramic, alumina, fibers on a nanometric scale. The invention is also directed at the preparation and formation of balloons using specifically carbon nanotubes, clay platelets and ceramic fibers.
2. Description of the Prior Art
Angioplasty addresses the problems related to partially or fully obstructed blood vessels. Angioplasty balloons have been used by invasive cardiologists since the 1970s when Andreas Grunzig reported his data on reopening the occluded coronary arteries of five patients and that these arteries remained patent, open, allowing blood flow for six months or longer. The Grunzig procedure involved the introduction of a high-pressure angiographic catheter with a deflated or collapsed polymer balloon attached to its distal portion. Once the catheter is positioned within the occluded range (lesion) in the artery under fluoroscopic control, the balloon is pressurized, typically by injecting a fluid. The pressure in the balloon exerts pressure on the surrounding obstructive structures and enlarges the lumen, which results in an increase in blood flow.
Subsequently, the balloon is depressurized until it collapses and it then can be withdrawn from the obstructed site where circulation has been restored through this maneuver.
As angioplasty, the reforming of blood vessels, has gained acceptance and replaced to a great extent the conventional coronary artery bypass graft procedure, a major surgical intervention, the demands for the performance of the balloon catheter have increased. These demands include high strength balloons to withstand pressures on the order of 10 to 20 atmospheres. In comparison, the typical passenger car's tires are inflated to about 32 psi or slightly above 2 atm. above the ambient pressure. While the typical tire wall is a composite, the walls are reinforced by high strength weaves of polyester filaments or stainless steel wires embedded in a visco-elastic matrix, such as neoprene, the typical angioplasty balloon is made of a polymer film with its wall thickness in the vicinity of 0.001″ or 25 micrometers. Thus the stress in the angioplasty balloon is determined to be about ten times higher internal pressure in a wall that is about one two hundredth thick, hence its stress is estimated to be 2000 times greater than in a tire. Both tires and balloon catheters encounter similar excess strain when they press against sharp objects. Automobile tires are usually designed to be puncture-proof when being pressed against a sharp nail; they are to allow the metal tip to penetrate while forming at least a temporary seal around it. The balloon catheter's wall may be exposed to the sharp edges of crystalline deposits. The ratio of the wall's thickness to the length of the puncturing object is much more favorable for the tire than for the angioplasty balloon, hence the balloon must exhibit great resistance to cuts by the sharp edges of the crystalline deposit in a plaque, the obstructive body in the vessel. Thirdly, both the automobile tire and the angioplasty balloon must be able to withstand overinflation without excessive plastic flow or rupture. In case the plaque to be broken by the inflated balloon is hard, dense and strong, the physician often attempts to inflate the balloon above its nominal pressure to increase the balloon's diameter in a predictable manner. For such events it is important to know the relationship between the desired additional diameter for the balloon above the rated value and the pressure necessary to achieve that.
When monomers in the general family of amides are polymerized to form polyamides, or Nylons, the strength of the balloon made from such materials is usually not isotropic, because the polymer chains are oriented to be parallel to the axis of the inflated balloon. The reason for this is rooted in the way the balloon is designed to fail in case it is overinflated beyond its failure stress level. The balloon is expected to split parallel to the catheter's axis to enable the withdrawal of the fractured balloon without leaving any portion behind. Such debris would typically require surgical removal that is possibly as traumatic as the bypass procedure that the angioplasty was expected to avoid.
The balloons, unlike tires, must also have lubricious surfaces and must be chemically inert. Polyamide or Nylon films generally meet these requirements.
Films made with certain polymer films containing either carbon nanotubes or clay platelets offer greater strength for the balloon without sacrifice of the viscoelastic properties necessary for over-inflation.
Carbon nanotubes were discovered by accident by Sumio Iijima, in 1991, in soot. Their properties have been studied extensively and the strength, flexibility and conductivity of the individual nanotubes have been measured with remarkable results. While singly, the nanotubes, characterized by approximately one nanometer diameter (10−9 m or 0.001 micrometer) have found few practical uses, they have been considered as the high strength component in composite materials. The physical characteristics of long nanotubes, which may reach a micrometer in length, suggest their use for conducting elements between semiconductor gates. Electron microscopists have observed carbon nanotubes with single or multiple but concentric cylindrical walls.
One of the factors which have delayed the use of nanotubes in composite materials is their extremely high cost. Some of this cost is associated with the difficulties in producing the nanotubes, typically from arc discharges. Another reason for the high cost is that often the electronic application requires a degree of uniformity in tube length and in the number of layers. The cost of a gram of uniform nanotubes has been in the range of $100 to over $1000. While this cost is a deterrent, the quantities of nanotubes needed for the manufacturing of angioplasty balloons are very small. For instance, a typical balloon may have 25 micrometer wall thickness and an equivalent diameter of 3 mm and length of 30 mm. The volume of material in such a balloon is merely 7 mm3. Further assuming that the volume ratio of carbon nanotubes to the volume of material is not more than 0.3, the volume of carbon nanotubes required is about 2 mm3. The mass of carbon in the balloon, assuming that the density is about 2 grams/cm3, is about 4 milligrams and its direct cost at $100 per gram is $0.40 per balloon.
As the cost of producing carbon nanotubes is declining, the composite material is economically practical in such demanding and relatively cost insensitive applications as coronary angioplasty.
Balloon fabrication has been extensively addressed in a number of US patents. For example, U.S. Pat. No. 5,868,704 Balloon Catheter Device (Issue date: Feb. 9, 1999, filing date, Jun. 26, 1996) by Campbell, Laguna and Spencer, assigned to W. I. Gore & Associates, describes composite balloon materials where the components are polymers, typically porous polytetrafluorethylene (PTFE) films combined with an elastomer to achieve some of the properties described above. One particular embodiment involves helically wound ribbons progressing in opposite directions to each other with specific pitch between the adjacent turns of the ribbon. The layers are thermally bonded to each other.
U.S. Pat. No. 5,506,049 of Swei and Arthur, assigned to Rogers Corp. is titled Particulate Filled Composite Film and Method of Making Same, and teaches the fabrication of films made with fluoropolymers filled with small particles for use as dielectric substrates.
U.S. Pat. No. 4,330,587 of Woodbrey, assigned to Monsanto Co. is titled Metal-Thermoplastic-Metal Laminates, teaches the fabrication of films which may be formed easily and exhibit high tensile strength. The core layer is polyamide or polyester sandwiched between aluminum alloy layers. This patent addresses applications calling for relatively thick layers where the core layer is between 0.01 and 0.09 inches (0.25 mm to 2.3 mm), with metal coatings of 0.002 to 0.0085 inches (50 to 210 micrometers).
U.S. Pat. No. 5,587,125 of Roychowdhury, assigned to Schneider (USA) Inc. is titled Non-Coextrusion Method of Making Multi-Layer Angioplasty Balloons, and teaches the fabrication of composite cylinders by fusing concentric tubes which then undergo blow molding.
U.S. Pat. No. 5,691,015 of Tsukamoto and Shimizu, assigned to Aicello Chemical Co. in Japan, is titled Composite Film Bags for Packaging, also teaches the fabrication of composite films, but those are used for making large bags suitable for storing chemical agents for agriculture where the outer film may be peeled off and the inner film is water soluble. When it is buried in soil, the film dissolves in water to release the agent.
U.S. Pat. No. 5,746,968 of Radisch, Jr., assigned to Interventional Technologies, Inc. is titled Method for Manufacturing a High Strength Angioplasty Balloon, and presents a method to increase the strength of a polymer balloon by special processing that results in directional orientation of the polymer chains using overstretching the balloon at an appropriate temperature. The method is claimed to preempt pinholes arising from the stretching steps. The balloon is not a composite.
U.S. Pat. No. 5,270,086 of Hamlin, assigned to Schneider (USA), with the title Multilayer Extrusion of Angioplasty Balloons, and presents a method to fabricate multiplayer balloons by co-extrusion, which have stable dimensions when stretched by pressurization.
U.S. Pat. No. 5,647,848 of Jorgensen, assigned to Meadox Medical, Inc., with the title High Strength Low Compliance Composite Balloon for Balloon Catheters, presents an elastomeric film that is restricted in its expansion under pressure by a constraining structure of filaments of high strength polymers such as Aramide, polyethylene or steel, carbon and so on. The result is a balloon strengthened against overexpansion by the helical filaments wound counter to one another.
Nylon 12 based nano-composites with low percentages of loading, on the order of a 2–5 percent, have achieved a significant (65%) decrease in the composite's flexural modulus and an even more significant (135%) increase in elongation. These properties seem to be needed for future angioplasty balloons as well as for stent deployment systems.